Rubber product and process of preparing the same



Pateated Se t. 8, 1931 urea, STATES PATENT OFFICE ERNEST n, snrnewarnaorWILMINGTON, DnLAwAnE, Assrcnon T 1:. I. no ron'r DE NEMOURS & COMPANY,OF WILMINGTON, DELAWARE, A CORPORATION on DELA- WARE RUBBER PRODUCT ANDPROCESS PREPARING THE SAME No Drawing.

This invention relates to a process for heat I treating rubber andsimilar materials and to the products obtained thereby, being more.

particularly directed to the use of certain materials whose function isto prevent scorch 5 ing in the presence of powerful organicaccelerators.

It has long been known that litharge, when i used in small quantities inconjunction with accelerators of the above mentioned type, has theefi'ect of reducing their activity at temperatures that are encounteredduring factory processing and litharge is therefore widely used for thepurpose of preventing scorching in the presence of such accelerators.

5 Litharge is, however, objectionable because it" is converted duringvulcanizationto lead sulfide which is black and the rubber compound istherefore darkened.

The prim'ary'object of the present invention u is therefore toprovide'means of retarding or inhibiting the action of powerful organicaccelerators under conditions that are encountered during factoryprocessing withoutinhibiting their action at the higher temperaturesthat are used for vulcanization and without adversely affecting thequality of the finished product. Anotherobject of the invention is toprovide means of preventing scorching-when active organic acceleratorsdo not tend to materiall are used, without using a material that willappreciably darken or discolor the vulcanized rubber. 4

I have found that various materials, which darken the rubber as doeslitharge and whic have not been heretofore employed in this connectionare excep tional retarding agents. Thus, cadmium ox-' ide, cadmiumhydroxide and the cadmium,

w salts, particularly the salts'of weak acids such as the salts of fattyacids, as cadmium oleate, cadmium stearate and cadmium acetate, as wellas various other compounds of cadmium are very effective in preventingthe scorching a of rubber mixescontaining accelerators of Applicationfiled April 12, 1929. Serial R0. min p the general type described below.Obviously other compounds than the preferred cadmium compounds such asmetallic oxides and salts which do not combine with sulfur to form blackor dark coloredproducts may also be trioxide.

I have discovered that the new retarding agents may be employed withadvantage in connection with many accelerators. One of having thischaracteristic is the group comprising the derivatives,- ofdithiocarbamic acid, that is, accelerators containing the group Theprincipal commercial accelerators falling when used in conjunction withaccelerators derived from the reaction of carbondi- -sulfide with analdehydeamine condensate such for example as the well known acceler- '55the most important groups of accelerators ators derived fromaniline-butyraldehyde and carbon bisulfide.

. In order to better disclose the invention in detail, examples aregivenbelow of'specific applications of the new retarding agents. Since thecompounds of-cadmium represent the preferred class of retarding agentsthe invention will hereinafter be'disclosed with more particularreference to these compounds butit is to be distinctly understood thatthe invention is not to be limited thereto. The accelerators employed inthe examples illustrate the invention as applied to accelerators of thesubstituted thiurammonosulfide type and to accelerators of the classderived from the reaction of carbon disulfide with an aldehyde aminecondensate. Accelerators of these two types, and more especially thesubstituted ,thi'urainmonosulfides, since the latter have been found tobe peculiarly adapted for use with cadmium compounds, represent thepreferred classes of accelerators and therefore have been chosen forpurposes of il lustrat on but, as already made clear, .varlous Example 1The following formula illustrates the man'- ner of using cadmiumcompounds in conjunction with tetramethylthiurammonosulfide:

Smoked sheets 100. 0 Zinc oxidenu '5. 0 Sulfur 2. 5Tetramethylthiurammonosulfide-4-- 0. 2

The table shown below gives the physical properties of this compoundwhen vulcanized for various periods of time at 227 F. and at 259 F., noretarding agent being used. Italso shows the efi'ect of adding 0.02% ofcadmium oxide to the compound and the effect of adding cadmium oleate,cadmium acetate, cadmium hydroxide and cadmium sulfide. The four last.mentioned compounds were used in molecularly equivalent quantities, thatis, in such amounts that the actual cadmium content 1s the same.

" Per cent Stress at Retarding agent Cure 500% 5 5 l elongation reng mmat break Lbs. per Lbs. per 811.511. sq. in. None x 227 F. 325 3,425, 83590 x 227 F. 725 3,900 745 15: 259 F. 150 2,700 870 20 *1 259 F. 375 3,225 785 30 x 259 F. 600 3,250 730 Cadmium oxide I 0.02% 60 x 227 F. Doesnot cure. 90x 227 F. 1.50 3,475 780 15 x 259 F. 175 2, 600 920 20 x 259F. 275 3,300 865 30 X 259 F. 425 3, 225 780 Cadmium oleate .Ofil3% 60 ii227 F. Does not euro.

90 227 F. 475 3,600 770 15 X 259 F. (150 2,100 875 20 x 259 F. 400 3,525820 30 x 259 F. 550 3, 350 770 Cadmium acetate 0.0599% 60 x 227 F Doesnot euro.

90 x 227 F. 325 3,350 850 15 1: 259 F. Does not cure. 20 x259" F. 225 2,850 855 30 1: 250 F. 425 3,225 780 Cadmium hydrox- I ide 0.0228% 60 is227 F. Does not A cure.

90 x 227 F. 525 3, 950 780 15 x 259 F. 200 2, 475 805 20 x 259 F. 3503,275 820 30 x 259 F.- 525 3,425 770 Cadmium sulfide V 0.0225% 60 x 227-F. 225 2, 750 880 q 90 x 227 F. 525 3,625 765 15 x 259 F. 200 2,725 85020 x 259 F. 372 3, 300- 800 30 x 259 F. 600 3,775 775 227 F. isapproximately the maximum temperature to which rubber compounds aresubjected during mixing, calendering and other factory processes whichprecede vulcanization. It will be noted that the compound containing noretarding agent is well cured in 60. minutes at 227 F. However, whencadmium oxide, cadmium oleate, cadmium acetate or cadmium hydroxide areadded in the amounts indicated in the table the compound does not cureat all in 60 minutes at 227- F. It is also significant to note thatcadmium sulfide, which is not capable of reacting with hydrogen sulfide,does not function as a retarding agent of low temperatures inthe rubbercompound used for this test.

259 is a common vulcanizing temperature for compounds containing theaccelerator used in the above example. It will benoted that the cadmiumcompounds, with the exception of the sulfide, retard' the cure at 259 F.to a slight extent at the beginning of the cure only. This is clearlyshown by the physical properties of the 15 minute cures at 259 F. Thecorrect technical cure for this compound is approximately 20 minutes at259 F. and in the proportions employed none of the cadmium compounds,with the exception of the acetate, have an appreciable effect on thephysical properties of the compound when cured for 20 minutes at 259 F.The fact that cadmium acetate has a slightly greater retardingeflectthan an equivalent amount, of cadmium oxide is probably due 100 tothe influence of tlie acetate radical. By employing slightlylessbuantities of the acetate results similar to those obtained from theother agents employed may be obtained.

In other words, the above example clearly indicates that cadmiumcompounds of the type described retard the vulcanization of rubbercompounds containing this accelerator at theinception of the cure andespecially at low temperatures but they have no. in'fiuence on the timerequired to produce a well cured stock at 259 F. Neither do they aiiectthe physical properties of the cured stock.

Example 2 Smoked I sheets".- 1O

Zinc oxide 5. 0 Sulfur. 3. 0 Anilinecarbondisulfide butyraldehyde 1. O

The table below showsthe results of physical tests on this compound whenvulcanized at 227 F. and at 259 F, also tests on 130 Per cent Stress atRetard!!! g agent Cure 500%91011- 23,2 2? 9 gation g M break Lbs. perLbs. per sq. m. sq. 111.

None. 45 x 227 F. t 125 l, 550 925 60 x 227 F 125 1, 075 910 75 x 227 F175 2, 250 000 90 in 227 F 200 2, 025 870 10 x 259 F 200 2, 200 855 20 x259 F 350 3, 150 S20 30X259 F 450 3,450 800 Cadmiumoxide 0.25% 45 x 227F Does not 15 cure. I

60 K 227 F 700 920 75 1: 227 F 50. 800 800 n, 9021227")? 50 1,675 9 5510 x 259 F. 75 1,150 910 X 259 F. 225 2, 325 875 30 x 259 F. 225 2, 900855 20 It will be noted ,that the rate of vulcanization of the compoundcontaining cadmium oxide is much slower at 227 F. than that of thecompound in which no retarding agent was used. The compound containingcadmium oxide gave a good cure in minutes at 259 F. although the rate ofcure at 259 F. was somewhat less than that of the compound containing nocadmium oxide.

ie percentage of cadmium oxide used in these tests was rather high. Aslightly lower percentage should be used in case one desires to retardthe cure of a rubber compound containing this accelerator at thetemperatures encountered during factory processing but to avoidretarding the cure at 259 F. The I percentage of any acceleratorrequired for any rubber compound must be determined in each case, as iswell known, by the nature of the other ingredients of the compound andthe conditions of vulcanization. In the same way, the percentage of ananti-scorching compound such as cadmium oxide is dependent upon thepercentage of accelerator, the nature of the other ingredients of thecompound. and the conditions under which the compound is to bevulcanized. The optimum proportions can bedetermined in any given 0instance by a simple experiment. 1

invention to a typical compound containing more than one accelerator.

Smoked sheets 100.0 Zinc oxide 5.0 Sulfur 2.0Tetramethylthiurammonosulfide 0.2 But-yraldehyde-aniline-carbondisulfidc 0.2

l I Strep: 32 2 Per cent- Retardlng Tensile elongaagent strength tion at400% 600% break Lbs. per Lbs. per Lbs. per sq. in. sq. in. sq. in. None30 X 227 F. f 225 1, 000 3, 500 800 45 x 227 F. 375 1, 875 3, 075 730 60x 227 F. 475 2, 250 4,075 600 10 x 250 F. 225 1, 050 3, 550 730 20 x 250F. 450 1, 950' 4, 100 725 30 )1 250 F. 400 1, 800 3, 525 710 Cfldfliill"oxide, 0.02%... 30 x 227 F. 75 325 1, 975 835 45 x 227 F. 250 1, 075 4,025 810 60 x 227 F. Y 350 1,750 4, 475 755 10 x 259 F. 150 550 3,100 86520 :1 250 F. 375 l, 4, 100 755 30 x 250 F. 375 1, 000 3, 625 720 Cadmiumoxide, 0.05%... 30 x 227 F. Does not cure.

' 45 x 227 F. 125 450 2, 575 S05 00 x 227 F. 225 025 3, 825 820 10 X 250F. 75 350 2, 425 900 20 x 250 F. 275 1, 225 3, 7 760 30 x 250 F. 400 1,650 3, 675 725 It will be noted that the compound containing 0.05 partsof t'admium oxide does not cure at all 111 30 minutes-at 227 F. Theonccontaining 0.02 parts is badly undercurcd under those conditionswhilethe one containingno cadmium oxide is very well cured. In 45 minutes at227 F. the compound containing 002 parts of cadmium oxide is well curedbut the one containing 0.05 parts of cadmium oxideis under-cured. In 60minutes at 227 F. all. three compounds are well cured. When of cadmiumoxide' vulcanization is conducted at 259 F'. we find that cadmium oxidehas a slight retarding effect. However, even a compound containmg 0.05%of cadmlum oxide is well cured 1n 30 minutes at 259 F the physicalproperties being practically the same as those of the compoundcontaining no retarding agent at all.

The examples given above illustrate the effect of adding cadmiumcompounds to a wide variety of.difi'erent types of rubber compounds. i

Milling tests show that the compounds con- -tainin g the new retardingagents as cadmium oxide-czidminnr oleate, cadmium acetate and cadm1umhydroxide have much less tendency to scorch during the milling processthan has the compound containing no retarding agent. This effect is notsusceptible of mathematical expression but can readily be observed byanyone who is experienced in the milling of rubber. pounds retardvulcanization at 227 F. also clearly indicates that they tend to preventscorching during milling, calendering, etc.

It is obvious from this discovery that there are a multitude of cadmiumcompounds which can be used in place of the preferred compoundsenumerated above. In fact, the cadmium salts of most acids maybeemployed, as, for example, cadmium stearate which gives the same resultsas cadmium oleate, and cadmium sulfate, which, although it is lesseffective than the cadmium salts of Weak acids, when used in somewhatlarger amounts fully answers the purposes of the invention. It is quiteclear from my experience that any cadmium compound which has the facultyof reacting with hydrogen sulfide or other forms of labile sulfur underthe conditions encountered in a rubber mix, will prevent scorching whenused in conjunction with accelerators of the type oftetramethylthiurammonosulfide.

The amount of cadmium oxide or othen cadmium compound normally requiredis approximately that used in the experiments described above, that is,about one-tenth as much cadmium oxide as oftetramethylthiurammonosulfide or a molecularly equivalent .amount ofsuch other cadmium compound as it may be desired to use. The proportionused, however, may vary up or down from that amount over rather widelimits, depending upon the nature of the other ingredients of thecompound and the temperature at which it is desired to vulcanize it.

As many apparently widely different embodiments of this invention may bemade without departing from the spirit and scope thereof, it is to beunderstood that I do not limit myself to the foregoing examples ordescription except as indicated in the following pa'tent claims:

I claim:

1. The process of treating rubber which comprises subjecting a rubbermix containing an accelerator of the class consisting of compoundscontaining the group I N-i JS and a cadmium compound which is adapted toreact with hydrogen sulfide and other forms of labile sulfur toconditions which would tend to cause scorching in the absence of saidretarding agent.

2. The process of treating rubber which comprises subjecting a rubbermix (containing sulfur, an accelerator of the class consisting ofcompounds containing the group 8 N-ti-s and a retarding agent of thegroup consisting The fact that these cadmium com-' of cadmium oxide,cadmium hydroxide and the cadmium salts of the fatty acids) toconcomprises subjecting a rubber mix (containing sulfur, an acceleratorof the substituted 'thiurammonosulfide type and a retarding agent of thegroup consisting. of cadmium oxide, cadmium hydroxide and the cadmiumsalts of the fatty acids) to conditions which would tend to causescorchingin the absence of said retarding agent.

4. The process of treating rubber which comprises subjecting a rubbermix (containing sulfur, an accelerator of the substitutedthiurammonosulfide type and a compound of the class consisting'ofcadmium oxide, cadmium' hydroxide, cadmium stearate and cadmium oleate)to conditions which would tend to cause scorching in the absence of saidcadmium compound. j

5. The process of treating rubber which comprises subjecting a rubbermix (containing sulfur, tetramethylthiur'ammonosulfide and a retardingagent of the group consisting of cadmium oxide, cadmium hydroxide andthe cadmium salts of the fatty acids) to conditions which would tend tocause scorching in the absence of said retarding agent.

6. The process of vulcanizing rubber which comprises subjecting a rubbermix (containing sulfur, an accelerator of the substitutedthiurammonosulfide type and a retarding agent of the group consisting ofcadmium 0x i'de, cadmium hydroxide and the cadmium salts of the fattyacids) to conditions which would tend to cause scorching in the absenceof said retarding agent and thereafter heat ing to a sufficiently hightemperature to eifect vulcanization.

7. A rubber product obtained by subjecting a mix (containing sulfur, asubstituted thiurammonosulfide and a retarding agent of the groupconsisting of cadmium oxide, cadmium hydroxide and the cadmium salts ofthe fatty acids) to conditions which would tend to cause scorching in.the absence of said retarding agent and subsequently vulcanizing thecompound.

8. A rubber product obtained by subjecting a mix (containing sulfur,tetramethylthiurammonosulfide and a retarding agent of the groupconsisting of cadmium oxide, cadmium hydroxide and the cadmium salts ofthe fatty acids having between two and eighteen carbon atoms) toconditions which would tend to cause scorching in the absence of saidretarding agent and subsequently Vulcanizing the compound.

9. The product of claim 8, said product being heated to a temperaturenot over 227 F. prior to the vulcanizing operation.

In testimony whereof, I aflix my signature.

ERNEST R. BBIDGWATER.

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